Vehicles such as electric vehicles and hybrid vehicles driven by electric motors are equipped with high voltage batteries. Direct-current power outputted from the high voltage battery is not only supplied to the electric motor but also converted in voltage by a power supply device (DC-DC converter) and then supplied to loads such as various electrical components and a low voltage battery of the vehicle. The power supply device is generally housed in a case formed using material having conductivity such as metal. By fixing this case to the chassis (frame) of the vehicle with metal screws or the like, the power supply device is grounded via the case and the chassis to a vehicle ground shared among other on-vehicle electrical equipment. Further, a ground strap cable for passing load current therethrough during power supply to the loads is connected between the power supply device and the chassis.
When the fixing of the case to the chassis is inadequate, or the connection of the ground strap cable is inadequate in the above-described power supply device, grounding may become defective. In that case, an unexpected large current may flow through an electric wire that connects the power supply device and the high voltage battery during power supply to the loads, causing a problem such as heat generation or damage. Thus, in order to safely supply stable power while preventing such a problem, it is required to monitor the ground connection of the power supply device.
With respect to monitoring the ground connection of electric equipment mounted on a vehicle, PTL 1, for example, discloses a technology in a charging device for charging a storage battery by converting alternating-current power from a power source to direct-current power, in which a voltage potential difference between an electric wire for charge output connected to the storage battery and an electric wire for connection to a vehicle body is detected, based on which a leakage resistance measure between the storage battery and the vehicle body is checked. PTL 2 discloses a technology in which the connection between a signal ground wire connected to the outside of a device and a power ground inside the device is cut off by a switch during operation of the DC-DC converter, and by measuring a voltage potential difference of the power ground relative to the signal ground at that time, the state of connection between the power ground and the device case is checked to monitor the ground connection.